This invention relates to apparatus and methods for monitoring and controlling the cooling system of power transformers.
Power transformers designed to distribute large amounts of power, such as substation and distribution class power transformers generally include cooling systems to remove heat generated when large loads are applied to the transformers (i.e., when large currents are drawn from and through the transformer). The cooling systems are designed to remove heat to help keep the transformer and its components below predetermined critical temperatures. Maintaining the transformer temperature below a critical value enables the transformer to handle a designed load capacity or to increase the power handling capability of the transformer.
The cooling systems may include cooling fans to circulate air over the transformer. Alternatively, the transformer may be contained within a liquid (e.g., oil) filled tank with oil pumps being used to circulate the fluid through radiators attached to the tank and cooling fans circulating air over the radiators. The operation of the cooling system is vital for the transformer to deliver its designed power capacity. If the cooling is compromised, the transformer temperature may rise above desired values. Such a rise in temperature may result in the outright failure of the power transformer and at a minimum will result in some damage and an accelerated loss of life. That is, over time excessive heating will reduce transformer life and lead to premature failure which will affect the ability of a utility company to supply uninterrupted supply of power to its customers and will cost the operating utility significant replacement costs.
Problems with prior art systems may be explained with reference to FIGS. 1, 1A and 2, which show a housing 100 enclosing a power transformer 120. As is known in the art, the primary and secondary windings of the transformer have some resistance (R). As current (I) flows through the windings, heat is generated which is a function of the winding resistance multiplied by the square of the current (i.e., I2R). A considerable amount of heat may be generated by, and within, the power transformer, particularly when the load is increased and more current flows through the transformer's primary and secondary windings.
The heat generated within the transformer causes a rise in the temperature of the windings and in the space surrounding the windings and all around the transformer. When the temperature rises above a certain level many problems are created. For example, the resistance of the (copper) transformer windings increases as a function of the temperature rise. The resistance increase causes a further increase in the heat being dissipated, for the same value of load current, and further decreases the efficiency of the transformer. With increased temperature the transformer may also be subjected to increased eddy current and other losses. The temperature rise may also cause unacceptable expansion (and subsequent contraction) of the wires. Also, the insulation of the windings and other components may be adversely affected. Temperatures above designed and desirable levels result in undesirable stresses being applied to the transformer and or its components. This may cause irreversible damage to the transformer and its associated components and at a minimum creates stresses causing a range of damages which decrease its life expectancy.
It is therefore desirable and/or necessary to maintain the temperature of the power transformer below a predetermined level.
In FIGS. 1 and 1A the transformer 120 may be cooled by immersing the transformer in a liquid (e.g., oil) and having the liquid flow through pipes 110 extending through the radiators (e.g., 2 and 41). Pumps (not shown) may be used to circulate the liquid (oil) through the radiators where the liquid may be subjected to cooling by means of cooling fans 6 and 7. A bank of cooling fans 6 and 7 (three fans are shown in bank 6 in FIG. 1) may be used to selectively blow air, or any other suitable coolant, over radiators (e.g., 2 and 41) to cool the liquid as it passes through the radiators. FIGS. 1 and 1A show: (a) a sensor 42 designed to sense the winding temperature which is coupled to a winding temperature control module 4 having an indicator for displaying the transformer winding temperature; and (b) a sensor 82 designed to sense the top oil temperature coupled to a top oil temperature control module 8 with an indicator for displaying the temperature of the top oil. The signals from sensors 42 and 82 are processed by their respective modules. When predetermined temperature levels are reached, the cooling fans 6 and 7 are powered by signals generated by and within fan motor control modules 4 and 8 in response to the signals generated by temperature sensors 42 and 82.
FIG. 2 illustrates circuitry, which may be contained in a control cabinet 3 attached to housing 100, for applying power to the fan motors to drive the fans. Control module 4 includes means for processing signals from sensor 42 and to generate a command signal applied to a motor winding control circuit 421 which, in turn, functions to control (turn-on and turn-off) switch 6S which then applies power to the motors (FM1, FM2, FM3) of cooling fans 6A, 6B and 6C In a similar manner, control module 8 includes means for processing signals from sensor 82 and to generate a command signal to a motor winding control circuit 821 which, in turn, functions to control switch 8S which then applies power to the motors (FM4, FM5, FM6) of cooling fans 7A, 7B and 7C.
Admittedly, the prior teaches the use of cooling systems to protect a power transformer from excessive temperatures. However, a problem with known prior art systems, as illustrated in FIGS. 1, 1A and 2, is that, in the event the cooling system fails, the temperature limits will be reached and/or exceeded before any corrective action can be taken. For example, if fan control switch 6S or 8S fails and/or in the event that a fan motor fails, the cooling of the power transformer is partially or wholly compromised. There is no provision which indicates the failure of the cooling device until the rise in temperature exceeds given limits and an alarm is sounded. Due to the large mass of the transformer system (there is a large thermal coefficient), by the time an alarm is sounded and corrective action is taken, the temperature of the transformer and associated components may rise considerably above desired and or design limits resulting in damage to the system.
Clearly, the prior art does not address the problem which arises when malfunctions and failures of the cooling system are not detected early and quickly. The prior art also does not address the need to monitor the functionality of the cooling system components. These problems and other drawbacks present in the prior art are overcome in systems embodying the invention.